gssd/svcgssd: add support to retrieve actual context expiration

Add some plumbing so that the context expiration can be returned while
serializing the information.  Later patch(es) will actually get the
expiration and pass it down to the kernel.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>